Added a BaseDatabaseOperations.last_executed_query() hook, which allo…

…ws a database backend to specify how to get the last-executed query on a given cursor. Implemented it for the psycopg2 backend. This means that for psycopg2, the SQL statements in django.db.connection.queries will now reflect the exact SQL as sent to the server, instead of a naive and misleading string-interpolated version
git-svn-id: http://code.djangoproject.com/svn/django/trunk@6601 bcc190cf-cafb-0310-a4f2-bffc1f526a37